Certain measurement applications require measuring the wavelength or frequency, or related shifts, of a radiation source to very high levels of resolution over a relatively small wavelength range. Examples include high resolution interferometric type encoders, various non-contact profilometer sensors, applications in the telecommunications industry, and spectroscopy, as well as general laboratory applications. In addition, for many applications, the measurement must be conducted within a small space and at a low cost. Several methods are commonly used for wavelength measurement, including spectrometers, interferometers, and transmission through optical filters.
FIG. 1A shows a simple known measurement system 10 for measuring wavelength shift using an optical bandpass filter. The measurement system 10 includes an input incident beam 12, a bandpass filter 14, a filtered beam 16, and a power detector 18. The input incident beam 12 is filtered by the filter 14 to produce the filtered beam 16. In this application, a bandpass filter 14 is not strictly required, as any optical element having a non-negligible wavelength transmission dependence (i.e., wherein radiation behaves differently according to its wavelength) can be used. The power of the filtered beam 16 is detected by the power detector 18.
FIG. 1B shows the transmission spectrum for the bandpass filter 14. The filter 14 is characterized by a central wavelength λ0, as well as its full width half maximum (FWHM) wavelength Δλ. A point P is shown on the filter curve 20 at a wavelength X1 and a transmission level Y1. It can be seen that the point P is located on the steep part of the filter curve 20, and that slight changes in the wavelength can thus be sensed by measuring the change in the transmitted power, as is done by the power detector 18 of FIG. 1A. In this manner, once the filter curve 20 is established, the measurement system 10 of FIGS. 1A and 1B provides a simple configuration for determining a wavelength shift based on a transmitted power or intensity.
FIG. 2 illustrates a known measurement system 30 which offers certain improvements over the measurement system 10 of FIG. 1A. As shown in FIG. 2, the measurement system 30 includes a beamsplitter 34, a filter 38, and power detectors 42 and 46. An input incident beam 32 is split into two beams 36 and 44 by the beamsplitter 34. The first beam 36 is filtered by the filter 38 to produce the filtered beam 40. As before, in this application the filter 38 is not strictly required, as any optical element having a non-negligible wavelength transmission dependence can be used. The power of the filtered beam 40 is detected by the power detector 42. The power of the second beam 44 is detected by the power detector 46. By utilizing the outputs of the power detectors 42 and 46 to compute a ratio of filtered to non-filtered beam powers, deviations in the power in the incident beam 32 are nominally eliminated as error sources. In other words, in contrast to the measurement system 10 of FIG. 1A which was unable to differentiate between wavelength shifts and power source fluctuations, the measurement system 30 of FIG. 2 uses a power ratio signal which is insensitive to deviations in the incident power, and thus more reliably discriminates wavelength shifts.
FIG. 3A illustrates a known measurement system 50 which provides an alternative configuration for measuring wavelength shifts. Similar to the measurement system 30 of FIG. 2, the measurement system 50 utilizes the ratio between two power detectors to eliminate the incident power dependence. The measurement system 50 includes a beamsplitter 54, filters 58 and 66, and power detectors 62 and 70. An incident input beam 52 is split into beams 56 and 64 by the beamsplitter 54. The first beam 56 is filtered by the filter 58 to produce a filtered beam 60. The power of the filtered beam 60 is detected by the power detector 62. The second beam 64 is filtered by the filter 66 to produce a filtered beam 68. The power of the filtered beam 68 is detected by the power detector 70.
FIG. 3B illustrates two filter curves 80 and 82 which correspond to the filters 58 and 66, respectively. As shown in FIG. 3B, the filter curve 80 overlaps with the filter curve 82. In other words, the transmission spectrum of the filter 58 overlaps with the transmission spectrum of the filter 66. A point P2 is shown on the filter curve 80 at a wavelength X1 and a transmission power Y2, and a point P1 is shown on the filter curve 82 at the wavelength X1 and at a transmission power Y1. It will be appreciated that for wavelengths increasing from wavelength X1, the transmission power on the filter curve 80 is decreasing, while the transmission power on the filter curve 82 is increasing. Thus, the ratio between a transmission power Y1 corresponding to the filter 66, and a transmission power Y2 corresponding to the filter 58 is unique for a particular wavelength over the wavelength transmission spectrum that is shared by the two filters 58 and 66. By utilizing the outputs of the power detectors 62 and 70 to compute a ratio of filtered beam powers, deviations in the incident power may be largely eliminated. In addition, this configuration provides for improved sensitivity to wavelength changes.
However, either due to their inherent shortcomings described above, or due to their susceptibility to certain other errors described below, none of the systems described above are suitable for detecting wavelength with a very high accuracy, unless undesirable set-up and operating restrictions are imposed on their use. Thus, a need exists for a wavelength detection system and method that avoids such error susceptibilities, without the need for such undesirable restrictions.